The present invention relates to a method of processing of grey level image data for rendering in accordance with image type.
Original documents can be categorized into three different image types: 1) text and line art, 2) continuous tone, and 3) halftones. For optimal reproduction, each of these original types should be processed differently. Text and line art documents have little if any grey level information and hence require little or no preservation of grey scale information. Continuous tone and halftone originals, on the other hand require tone scale preservation to reproduce a large number of grey levels. In order to preserve the sharpness of text edges, a two dimensional FIR filter is applied to text and line art images. However, continuous tone and halftone images do not require as strong a filter. Halftone images typically require special processing to minimize moire patterns that can result from sample aliasing during the scan process.
Compound documents are documents which contain two or more of these image types within one page of an original. When such documents are scanned, image segmentation or image recognition technology is required to optimally process (in an automated manner without human intervention) each of the different image areas within a scanned image. Note that with human intervention, such an image can be optimally processed by designating areas of the image and how each area is to be processed. This method is not as productive as an image segmentation method.
Great strides have been made in image segmentation technology. Unfortunately, with as much progress as has been made, this technology is not always 100% accurate in its classifications. As a result, an alternative to image segmentation technology is to use a single processing method which although not optimal for each of the image types, results in image quality which is quite good. Such a method is referred to as a robust default rendering mode.
Binary error diffusion has proven to be a robust default mode. It""s high frequency response allows for good preservation of text edges and pictorial details. It""s ability to reproduce many levels of grey allows for good pictorial reproduction. The somewhat random nature of the dot creation provides for good rendition of halftone originals.
Grey scale writing systems provide the potential for significant enhancement of image quality for electrophotographic digital copier systems. Grey scale writing systems can utilize either a laser or LED exposure system. Different dot sizes or dot densities can be formed by modulating either the time or density of the exposure system.
Given a grey scale writing system, multibit error diffusion would seem to be a logical choice for default processing of compound documents. One would expect that grey pixels added along the edges of straight lines and curves of text could result in much smoother line edges. One would also expect that highlight regions of a pictorial would have a softer first step due to the ability to expose dots at a fraction of those exposed in a binary system. And one would also expect a significant improvement in the number of grey levels and tone scale reproduction of continuous tone and halftone originals. But due to the nature of the electrophotographic process, not all of these expectations are realized.
The inventors have noted that with multibit error diffusion, in general, and two bit error diffusion, in particular, certain dot patterns created by the multibit error diffusion algorithm result in poor image quality. This has been found to occur in the absence of a strong (fully exposed) dot. The result is that mid grey continuous tone regions are reproduced with a very course grainy structure. The same problem is exhibited with very high frequency halftone originals. The reason for this is the inherent inability of the electrophotographic process to expose and print continuous tone grey scale. Consider as an example a two bit writing system in which the possible exposure settings are 0, 1, 2, and 3 with 3 being most exposure and resulting in the largest or most dense dots. If a small region is exposed which contains all 1""s, the result is a flat field region with a grainy appearance. This is a well recognized problem with electrophotographic systems. The reason is grey reproduction is best when there is structure in which dots are grown in clustered groups. However, the very nature of error diffusion is to disperse dot growth based on its mean preserving calculations.
In binary error diffusion, all pixel decisions result in a stable maximum exposure dot. With the exception of highlights, reproduction of mid grey continuous tone flat fields is not a problem. But binary error diffusion, even at resolutions of 600 dpi, creates text and line edges which are ragged in comparison to multibit error diffused text and line art. The quality of text and line reproduction can be improved by adding grey level along the edges. Reproduction of highlights is also a problem with binary error diffusion due to the fully exposed isolated dots.
The basis of the invention is to take advantage of the mean preserving nature of error diffusion.
In accordance with the invention, there is provided a method of processing image data of pixels to be rendered comprising providing image data representing a pixel to be rendered; establishing a criteria for processing the pixel in accordance with binary error diffusion or multibit error diffusion; and providing a decision as to whether the pixel is to be rendered in accordance with binary error diffusion or in accordance with multibit error diffusion; and rendering the pixel in accordance with the decision.